Digestive System Functions

Overview of Digestion

• Digestive system: breakdown of ingested food, absorption of nutrients, and passage of waste products
• Two main groups of organs: alimentary canal (nutrition) and accessory digestive organs

Mouth (Oral Cavity) Anatomy

• Vestibule: space between lips externally and teeth and gums internally
• Tongue: attached at hyoid and styloid processes, and by the lingual frenulum
• Frenulum: membrane that secures the tongue to the floor of the mouth, limits movement
• Tonsils: palatine tonsils and lingual tonsil

Pharynx

• Serves as a passageway for air and food
• Food is propelled to the esophagus by two muscle layers (peristalsis)

Esophagus

• Runs from pharynx to stomach through the diaphragm
• Conducts food by peristalsis (slow rhythmic squeezing)
• Passageway for food only (respiratory system branches off after the pharynx)

Stomach

• Acts as a storage tank for food
• Site of food breakdown
• Chemical breakdown of protein begins
• Delivers chyme (processed food) to the small intestine
• Produces 2-3L/day of gastric juice (HCl, enzymes, and mucus)
• Regulated by neural and hormonal factors

Small Intestine

• Site of nutrient absorption into the blood
• Muscular tube extending from the pyloric sphincter to the ileocecal valve
• Approximately 4-8 hours for food to pass through
• Subdivisions: duodenum (5%), jejunum (40%), and ileum (55%)
• Digestion: breaks down double sugars into simple sugars, completes protein digestion, and aids in fat digestion
• Absorption: water is absorbed along the length of the small intestine

Large Intestine

• Absorbs water and eliminates indigestible food from the body as feces
• Does not participate in digestion of food
• Structures: ileocecal valve, cecum, appendix, colon (ascending, transverse, descending, and sigmoidal), rectum, and anus
• Functions: absorption of water, elimination of indigestible food, and production of vitamin K and B
• Propulsion: slow, powerful movements (mass movements) occur three to four times per day

Accessory Digestive Organs

• Salivary glands: produce saliva that aids in chemical digestion

• Pancreas: produces a wide spectrum of digestive enzymes that break down all categories of food

• Liver: produces bile, detoxifies drugs and alcohol, degrades hormones, produces cholesterol, and plays a central role in metabolism### Anatomy of the Liver

• The liver consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament.

• It is connected to the gallbladder via the common hepatic duct.

Carbohydrate Metabolism

• If the body has an abundance of glucose, it is converted into glycogen (glycogenesis).
• If the body lacks sugar, the liver breaks down glycogen into sugar (glycogenolysis).
• Glucose is broken down, releasing chemical energy to form ATP during cellular respiration.

Fat Metabolism

• Most fatty acid metabolism occurs in the liver.
• Fats are broken down into acetic acid, and then oxidized to form CO2, H2O, and ATP.

Protein Metabolism

• Amino acids are used to make ATP only when proteins are excessive or carbohydrates and fats are unavailable.
• Amino acids are oxidized, and ammonia (NH3) is secreted; the rest enter the citric acid cycle.

Nutrition

• A nutrient is a substance used by the body for growth, maintenance, and repair.
• Categories of nutrients:
  • Carbohydrates (simple sugars, starches, fiber)
  • Lipids (triglycerides, phospholipids, fatty acids)
  • Proteins (amino acids)
  • Vitamins
  • Minerals (7 essential minerals: Ca, P, K, S, Na, Cl, Mg)
  • Water

Diseases and Disorders

• Gallstones occur when bile is stored for too long, and fat crystallizes.
• Jaundice occurs when bile enters the bloodstream, and tissues become yellow.

Processes of the Digestive System

• Ingestion: getting food into the mouth
• Food breakdown: mechanical and chemical digestion
• Food movement: peristalsis and segmentation
• Absorption: end products of digestion are absorbed into the blood or lymph
• Defecation: elimination of indigestible substances

Energy and Metabolism

• Energy intake = total energy output (heat + work + storage energy)
• Basal metabolic rate (BMR) is the amount of energy needed to perform essential life activities.
• Factors that influence BMR: surface area, gender, age, and thyroid function.

Urinary System

• Functions:
  • Elimination of waste products
  • Maintenance of blood composition
• Organs: kidneys, ureters, urinary bladder, and urethra

Kidney Structure

• Location: high on the posterior abdominal wall
• Coverings: fibrous capsule, adipose capsule, and renal fascia
• Regions: renal cortex, renal medulla, and renal pelvis

Nephrons

• Functional units of the kidneys
• Consist of renal corpuscle and renal tubule
• Responsible for forming urine
• Over 1 million nephrons in each kidney

Urine Formation

• Processes: filtration, reabsorption, and secretion

• Filtration: nonselective, passive process at the glomerulus

• Regulation of filtration rate: sympathetic nervous system reflexes and hormonal regulation### Renal Function and Urine Production

• Filtration pressure: hydrostatic pressure of blood forces substances through the capillary wall, while net filtration pressure is always positive.

• Hydrostatic pressure of blood is greater than the hydrostatic pressure of the glomerulus capsule and the osmotic pressure of glomerulus plasma.

• The sodium potassium pump reabsorbs 70% of sodium ions in the proximal convoluted tubule (PCT).

The Renin-Angiotensin Mechanism

• Renin is produced by juxtaglomerular cells in response to stimuli such as sympathetic stimulation, decreased blood pressure, and decreased sodium and chloride ions.
• Renin reacts with angiotensinogen to form angiotensin I, which is converted to angiotensin II by the angiotensin I converting enzyme (ACE).
• Angiotensin II acts to vasoconstrict efferent arterioles, increasing blood pressure and maintaining filtration rate.

Reabsorption and Secretion

• Reabsorption: water, glucose, amino acids, and needed ions are transported out of the filtrate into the peritubular capillary cells.
• Most reabsorption occurs in the proximal convoluted tubule (PCT), where microvilli cells act as transporters.
• Active transport of sodium ions occurs along the remainder of the nephron and collecting duct.
• Almost all sodium ions and water are reabsorbed.
• Secretion: hydrogen ions, potassium ions, creatinine, and drugs are removed from the peritubular capillaries and secreted into the filtrate.

Maintaining Water Balance

• Normal amount of water in the human body: 50% in young adult females, 60% in young adult males, 75% in babies, and 45% in older adults.
• Water is necessary for many body functions, and levels must be maintained.
• Distribution of body fluid: intracellular fluid (inside cells), extracellular fluid (outside cells), interstitial fluid, and blood plasma.

The Link Between Water and Salt

• Electrolyte balance affects water movement between compartments.
• Changes in electrolyte balance alter blood volume and blood pressure.
• Aldosterone regulates sodium ion content of extracellular fluid.

Urine Characteristics

• Composition: ~95% water, containing urea, uric acid, electrolytes, and amino acids (trace amounts).
• pH: 4.5 to 8.0, with acidic urine produced during protein-rich diet, starvation, and diabetes, and basic urine produced during bacterial infections and vegetarian diets.
• Sterile, slightly aromatic, and yellow due to the pigment urochrome.

Urinary System

• Ureters: slender tubes attaching the kidney to the bladder, 10-12 inches long and ¼ inch diameter.
• Urinary bladder: smooth, collapsible, muscular sac that temporarily stores urine.
• Urethra: thin-walled tube that carries urine from the bladder to the outside of the body, with a length of 3-4 cm in females and 20 cm in males.

Micturition (Voiding)

• Both internal and external urethral sphincters must open to allow voiding.
• The internal urethral sphincter is relaxed after the bladder stretches to ~200mL, and the external urethral sphincter must be voluntarily relaxed.
• Incontinence: inability to control micturition, and retention: inability to micturate.

Male Reproductive System

• Testes: primary sex organs that produce sperm and secrete androgens such as testosterone.
• Epididymis: comma-shaped, tightly coiled tube that matures and stores sperm cells.
• Ductus deferens: carries sperm from the epididymis to the ejaculatory duct.
• Urethra: carries both urine and sperm.
• Accessory organs: seminal vesicle, prostate gland, and bulbourethral gland.
• External genitalia: penis and scrotum.